In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two closed (endless) wires, fabrics or belts which press the web one from above and the other one from below against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular when the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single closed (endless) drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve and is subjected to negative pressure as it runs over the reversing cylinders in order to maintain the web on the wire. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
In so-called normal groups with single-wire draw, known in the prior art, the heated drying cylinders are placed in an upper row and the reversing cylinders or rolls are placed in a lower row below the upper row of drying cylinders, which rows are typically horizontal and parallel to one another. In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers, of the type mentioned above. In an inverted dryer group, the heated drying cylinders are placed in a lower row and the reversing cylinders or rolls are placed in an upper row above the lower row of drying cylinders.
It is known to those skilled in the art that if paper is dried one-sidedly, the result is a tendency of curling of the sheet. For example, when paper is dried by means of normal groups with single-wire draw from the side of its bottom face only, the drying is asymmetric and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, when viewed from above.
As known in the prior art, the tendency of curling of paper (or the tendency to curl) is already affected in connection with the web formation, in particular at the sheet formation stage (in, for example, the current assignee's former designated Sym-Former.TM.) by means of the selection of the difference in speed between the slice jet and the wire, by means of the choice of the former and its mode of running, and also by means of other running parameters. For example, in the case of copying paper, by means of unequalsidedness of drying in the after-dryer, a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying could be optimized. In the case of copying paper, the reactivity of curling, i.e., the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper, which is produced in connection with the web formation in the wet end.
The most recent technology related to the present invention in high-speed paper machines is based on dryer sections in which there is single-wire draw over the major part of the length of the machine and, with a view toward controlling the tendency of curling of paper, in practice, an inverted group is also almost always used in order to make the drying sufficiently symmetric in the z-direction. However, it has been found that an inverted group has certain obvious, inherent drawbacks in view of the runnability and the overall efficiency of the machine and in view of the profitability of the paper machine investment. Thus, from the point of view of the runnability of the paper machine, a dryer section fully supported over its entire length and based on the use of only normal groups with single-wire draw, without using any inverted groups, would be a highly justified solution. People skilled in the art have, however, not had the courage to introduce this solution in operation, because it has been considered that it would result in uncontrollable and unfavorable constructions from the point of view of the tendency of curling of paper.
One particular problem in the prior art dryer section constructions that include one or more inverted dryer groups is the removal of broke in the event of web breaks, because inverted groups are not self-cleaning by the effect of gravity.
With respect to the prior art related to the present invention, reference is made to the current assignee's Finnish Patent No. 91,900 (corresponding to U.S. Pat. No. 5,416,980 incorporated by reference herein), in which a method is described in the dryer section of a paper machine in particular for reducing the tendency of curling of paper. In this method, the paper web is dried by means of drying cylinders against whose heated faces the paper web is pressed by means of a drying wire. In the dryer section, groups of drying cylinders are used in which twin-wire draw and/or single-wire draw is/are applied. In this method, it has been considered advantageous that in the dryer section, substantially across the entire width of the paper web, hot water steam is fed, by whose means the strains that arise or tend to arise in the fiber mesh in the paper web are relaxed by means of heat and moisture in, or substantially directly after, the area of formation of the strains.
In the current assignee's Finnish Patent Application No. 963734 (corresponding to U.S. provisional patent application Ser. No. 60/030,693), a method is described for drying a surface-treated paper web or equivalent in an after-dryer of a paper machine as well as a dryer section of a paper machine for applying the method. With a view toward compensating for a tendency of curling of the paper web, in the after-dryer, the paper web is dried in a dryer group/groups making use of a normal single-wire draw. In connection with or after the drying, the paper web is treated by means of at least one device in order to compensate for a tendency of curling of the paper web, which devices are, for example, a steam box, a blower unit, a moistening device, and/or a soft calender.
Further, in the current assignee's Finnish Laid-Open Publication No. 98,387 (corresponding to U.S. patent application Ser. No. 08/705,059, incorporated by reference herein), a method is described for the manufacture of paper, in particular fine paper, to be surface-treated as well as a dry end of a paper machine that makes use of the method. The paper web, which has been dewatered by pressing, is dried in a forward dryer section, in which drying energy is applied to the paper web over the entire length of the forward dryer section asymmetrically in the z-direction from the side of the lower face of the web. This forward drying stage is carried out by means of a number of successively arranged groups with single-wire draw open towards the bottom on the support of the drying wire. In this manner, shrinkage of the web, which tends to take place both in the machine direction and in the cross direction with an increase in the dry solids content, is substantially prevented. In connection with a web break, the paper broke is removed downwards from the dryer groups open towards the bottom substantially by the force of gravity onto the broke conveyor placed underneath. Owing to the asymmetric forward drying, the paper web with a tendency of curling is passed into a finishing section, in which it is after-treated while it is, at the same time, moistened or worked plastically so that the tendency of curling that arose in it in the forward drying stage is eliminated. The after-dryer section may include groups with twin-wire draw and regulation of steam as well as steam boxes arranged to control the curl and infra and airborne web drying.
In the current assignee's Finnish Patent Application No. 964830 (corresponding to U.S. provisional patent application Ser. No. 60/032,405), a method is described for drying paper as well as a dry end of a paper machine. The method for drying paper comprises the following steps: the paper web to be dried is passed from the press section into a forward dryer section, in which the paper web is dried from the side of its bottom face in dryer groups that apply a normal single-wire draw, the forward dryer section comprising exclusively single-wire groups with normal single-wire draw, and from the forward dryer section the paper web is passed into a finishing section. In the finishing section, the paper web is coated/surface-sized by means of a coating/surface-sizing equipment, thereafter dried in an after-dryer section by passing through at least one dryer group that applies a normal single-wire draw. After the after-dryer section, the paper web is calendered in a calender and passed to a reeling station in which the paper web is reeled into a machine reel. The curling of the paper web is controlled by means of elements and/or by means of assemblies and combinations formed out of such elements in the area of the forward dryer section and/or the finishing section. The dry end of the paper machine comprises a forward dryer section and a finishing section, which finishing section comprises a coating/surface-sizing equipment, an after-dryer, a calender, and a reeling station. The dry end of the paper machine comprises elements and/or assemblies and combinations formed out of such elements arranged to control curling of the paper web in the area of the forward dryer section and/or the finishing section. The elements for controlling the curl of the web include, among other things, blowing of hot moist air through the wire in the forward dryer section, steam boxes employed in the after-dryer, a combination in which steam treatment by means of a steam box is combined with a cooling cylinder, a lower support belt or support wire in the after-dryer, twin-wire groups employed in the after-dryer, blowing through the wire in connection with at least one cylinder in the after-dryer, the use of a suitable cylinder-diameter ratio, atomizing of water against the web in the after-dryer, infrared boxes for treatment of the web before the calender, transfer of moist air from the forward dryer to the after-dryer to be blown against the web, and mechanical working of the web by means of a spreader bar.
With respect to the prior art, reference is also made to U.S. Pat. No. 5,557,860 which describes a dryer section including dryer groups with a normal single-wire draw and a moistening device arranged after the dryer groups and by means of which the curl of the web is controlled.